Annotations on return-overridden methods also end up on bridge methods

[Remi Forax]

On 11/21/2013 12:20 PM, Vicente-Arturo Romero-Zaldivar wrote:
> Hi Charles,
>
> Comments below.
>
> On 21/11/13 08:16, Charles Oliver Nutter wrote:
>> Hey there!
>>
>> Sorry if this has been asked before, but I've been trying to get JRuby
>> to compile properly on JDK8 javac and noticed a change.
>>
>> JDK8 generates bridge methods in cases where a child class overrides a
>> superclass method with a more specific return type. This may have been
>> present in JDK7 and lower, I'm not sure.
>>
>> What does seem to be different is that annotations attached to the
>> hand-written override are also being attached to the bridge method.
>>
>> An example from JRuby...
>>
>> RubyBasicObject defines op_equal:
>>
>> public class RubyBasicObject ... {
>> ...
>>      @Override
>>      public IRubyObject op_equal(ThreadContext context, IRubyObject 
>> obj) {
>>          return op_equal_19(context, obj);
>>      }
>> ...
>> }
>>
>> RubyMethod extends RubyObject which extends RubyBasicObject, and it
>> also overrides op_equal with a more specific return type:
>>
>> public class RubyMethod extends RubyObject {
>> ...
>>      @JRubyMethod(name = "==", required = 1)
>>      @Override
>>      public RubyBoolean op_equal(ThreadContext context, IRubyObject 
>> other) {
>> ...
>> }
>>
>> We use the JRubyMethod annotation to generate stubs for binding the
>> Ruby class's methods at boot time. These stubs are generated by
>> walking declared methods looking for the JRubyMethod annotation.
>>
>> However, on JDK8, we get an error like this:
>>
>> Exception in thread "main" java.lang.ClassFormatError: Duplicate
>> method name&signature in class file
>> org/jruby/RubyMethod$INVOKER$i$op_equal
>>
>> This is because JDK8's generated IRubyObject-returning bridge method
>> also has the JRubyMethod annotation.
>>
>> Here's the relevant entry in the constant pool:
>>
>> system ~/projects/jruby $ javap -v -cp core/target/classes/ 
>> org.jruby.RubyMethod
>> Classfile 
>> /Users/headius/projects/jruby/core/target/classes/org/jruby/RubyMethod.class
>> ...
>>    #122 = Utf8               RuntimeVisibleAnnotations
>>    #123 = Utf8               Lorg/jruby/anno/JRubyMethod;
>>    #124 = Utf8               name
>>
>> Here's the hand-written method plus RuntimeVisibleAnnotations:
>>
>>    public org.jruby.RubyBoolean
>> op_equal(org.jruby.runtime.ThreadContext,
>> org.jruby.runtime.builtin.IRubyObject);
>>      descriptor:
>> (Lorg/jruby/runtime/ThreadContext;Lorg/jruby/runtime/builtin/IRubyObject;)Lorg/jruby/RubyBoolean; 
>>
>>      ...
>>      RuntimeVisibleAnnotations:
>>        0: #123(#124=[s#149],#150=I#137)
>>
>> And here's the bridge method, which also acquires this annotation:
>>
>>            public org.jruby.runtime.builtin.IRubyObject
>> op_equal(org.jruby.runtime.ThreadContext,
>> org.jruby.runtime.builtin.IRubyObject);
>>              descriptor:
>> (Lorg/jruby/runtime/ThreadContext;Lorg/jruby/runtime/builtin/IRubyObject;)Lorg/jruby/runtime/builtin/IRubyObject; 
>>
>>              flags: ACC_PUBLIC, ACC_BRIDGE, ACC_SYNTHETIC
>>              ...
>>              RuntimeVisibleAnnotations:
>>                0: #123(#124=[s#149],#150=I#137)
>>
>> I'm not arguing that this is correct or incorrect behavior...merely
>> that it is a change, and it may bite others the same way it has bitten
>> us. I have patched our stub generator to skip bridge methods, but it
>> feels kinda wrong.
>
> This is not a bug but intended behavior, I implemented this as a 
> feature request, see [1]. Some users asked this as a nice to have 
> feature :)
>
> [1] https://bugs.openjdk.java.net/browse/JDK-6695379

while I agree that it's a feature not a bug, or even a bugfix if you 
consider that without that fix you have a way to bypass
JavaEE security frameworks that use annotations.
Anyway, it has to be documented in the compatibility notes because this 
change in the behavior
may impact some annotations processor or runtime that use the annotations.

>
>>
>> - Charlie
>

Rémi